Graphene is a sheet-like substance in which carbon atoms are arranged in a honeycomb pattern, with a thickness of approximately 0.334 nm. The conduction band and the valence band of graphene can approximate in symmetrical cones having their vertexes contacted with each other at a K point (Dirac point) in a Brillouin zone. Graphene is expected as a material of next-generation electronics since the Fermi level EF of pure single-layer graphene is at the Dirac point (EF=0 eV) and electrons near the Dirac point have a carrier mobility not less than 10,000 cm2V−1s−1.
In addition, graphene is also noted as a material of optical device application since graphene has a property of absorbing light in a wide wavelength range of from visible to infrared light.
It is known that graphene acts as a photodetector if holes and electrons generated by light absorption are extracted as a photocurrent before recombination by utilizing the high mobility of graphene and a built-in field in a graphene. In general, however, a photocurrent extraction efficiency of the graphene photodetector is low.
In addition, a graphene-based optical modulator is also known. In general, however, an extinction ratio of the graphene optical modulator is low.